Re: [PATCH v3] drm/i915: Optimistically spin for the request completion

Chris Wilson
2015-03-12T13:19:04Z

On Thu, Mar 12, 2015 at 01:14:30PM +0000, Tvrtko Ursulin wrote:
> On 03/12/2015 11:11 AM, Chris Wilson wrote:
> >This provides a nice boost to mesa in swap bound scenarios (as mesa
> >throttles itself to the previous frame and given the scenario that will
> >complete shortly). It will also provide a good boost to systems running
> >with semaphores disabled and so frequently waiting on the GPU as it
> >switches rings. In the most favourable of microbenchmarks, this can
> >increase performance by around 15% - though in practice improvements
> >will be marginal and rarely noticeable.
> >
> >v2: Account for user timeouts
> >v3: Limit the spinning to a single jiffie (~1us) at most. On an
> >otherwise idle system, there is no scheduler contention and so without a
> >limit we would spin until the GPU is ready.
>
> Isn't one jiffie 1-10ms typically?

1ms. I was just thinking of doing USECS_PER_SEC / HZ, then realised that
was a jiffie, hence the confusion. At any rate, it is still the minimum
we can trivially wait for (without an expensive hrtimer).
-Chris

--
Chris Wilson, Intel Open Source Technology Centre